The present invention relates to a method and a device for controlling a gas fill of a plurality of cylinders in an internal combustion engine having variable valve timing.
Conventional internal combustion engines have one or more camshafts for controlling the engine valves with a predefined lifting curve. The lifting curve of valve actuators of the engine valves is defined by the design of the camshaft. However, a defined lifting curve does not permit optimal operation of the internal combustion engine in each operating state, since in general, different operating states of the internal combustion engine require different lifting curves.
Because of the fixed lifting curve, control of a gas exchange, i.e. filling a gas into a cylinder and letting gas out, is possible only to a limited extent in a conventional internal combustion engine having cam-actuated valves, and cannot be optimized for all operating states. A gas fill denotes the charge of a combustion chamber of the cylinder with air, added fuel and, depending on the operating state and the type of internal combustion engine, a recirculated exhaust gas from the combustion.
The control of the gas exchange (also known as charge cycle) in a camless internal combustion engine differs in principle from this. In an internal combustion engine having an electrohydraulic, camless valve drive, the engine valve operations are flexible. The quantities of intake air and of residual exhaust gas in each cylinder can be controlled by varying the instant of the opening and/or closing of the intake and exhaust valves. While an electrohydraulic, camless valve drive offers more flexibility, on the other hand, disadvantages exist which do not occur in arrangements having mechanical camshafts.
In systems with fully variable valve timing, the lifting curve of the valve actuators exhibits certain tolerances from actuator to actuator. These deviations of the lifting curves are caused by tolerances of mechanical, hydraulic, magnetic or electrical components of the valve actuators. Furthermore, the lifting curves change due to different wear over time in an unequal manner.
However, different lifting curves produce filling differences between the individual cylinders. This leads to a deterioration in the smooth-running performance of the internal combustion engine, elevated noise emission, increased fuel consumption, and a greater stress on the moving parts of the internal combustion engine, which results in greater wear.
German Patent No.195 11 320 proposes the formation of cylinder-individual correction values for the solenoid-valve control of an electrohydraulic valve actuator. The fresh-gas and residual-exhaust-gas charge is ascertained from a fresh air measured individually for each cylinder and a combustion chamber pressure measured individually for each cylinder, as well as from a temperature, and is compared to pre-determined setpoint values. Thus, a balance of different lifting curves is achieved by suitable determination of the correction values.
To measure these quantities, German Patent No. 195 11 320 proposes using a single air-mass flow sensor in the intake flow and/or a single oxygen sensor in the exhaust-gas flow which is operated on a time-resolving basis. With the detected air quantity, the residual exhaust-gas portion can then be ascertained by offsetting against a combustion chamber pressure measured in a cylinder-individual manner, given a specific piston reference position and the temperature.
However, in the device disclosed in German Patent No. 195 11 320, it is necessary to ascertain the above-indicated measured quantities individually for each cylinder. Thus, it is necessary to arrange at least one combustion-chamber pressure sensor per cylinder. This is very costly, since a pressure sensor must be arranged with access to the combustion chamber of the cylinder. Moreover, the customary pressure sensors are temperature-sensitive, and output inaccurate measured values in response to changing temperatures.
German Published Patent Application No. 42 36 008 describes a cylinder-individual lambda closed-loop control for a system having variable valve timing. There, a lambda probe is used indirectly as filling sensor, in that the fresh-air charge of the cylinder is calculated back with the aid of the injection quantity. However, this device supplies inaccurate calculation values for the fresh-air charge, since because of the necessary rapid calculation, many influence factors such as a temperature of the internal combustion engine, a load status of the internal combustion engine, a combustion-chamber pressure and an ambient temperature cannot be sufficiently taken into account in the calculation. In addition, this device assumes injection valves with a very small tolerance in order to be able to calculate usable results for the fresh-air charge at all.
An object of the present invention is to specify a method and a device, which are simple and accurate, for controlling a gas fill of a plurality of cylinders in an internal combustion engine having variable valve timing.
In the method for controlling a gas fill of a plurality of cylinders in an internal combustion engine having variable valve timing, a multitude of sampling values of a detection signal of a filling sensor are ascertained by sampling the detection signal with a sampling rate. Furthermore, a first detection interval is determined for a first cylinder. In the following, those sampling values which are within the first detection interval for the first cylinder are summed up. This sum is designated as first sampling-value sum. In addition, the number of sampling values within the first detection interval is counted. This yields a first count value. A first air mass filled into the first cylinder is then ascertained by forming a quotient from the first sampling sum and the first count value.
The advantages attained with the invention are, in particular, that the first air mass filled into the first cylinder is ascertained in a simple manner based on one detection signal. This detection signal is the output signal of a filling sensor. Beyond the detection signal of the filling sensor, it is preferably not necessary in the method of the present invention to acquire further measured quantities such as a combustion-chamber pressure or a temperature. Furthermore, the method according to the invention supplies a precise gas fill, i.e. the first air mass filled into the first cylinder, even when inexact injection valves having great tolerances are used.
In one advantageous refinement of the invention, the first detection interval is variable.
Due to a change in the first detection interval, a selection may advantageously be made as to whether, for example, the detection signal of a filling sensor is evaluated upon opening of the valve or upon closing of the valve. It is thereby possible to determine whether the valve or the valve actuator exhibits more deviations or tolerances during opening or during closing, and whether the valve or the valve actuator has a defect.
In a further advantageous development of the invention, a second detection interval for a second cylinder of the plurality of cylinders of the internal combustion engine is determined. The sampling values within the second detection interval are then summed up for ascertaining a second sampling-value sum. Furthermore, the number of sampling values within the second detection interval are counted. The count value is designated as the second count value. An air mass filled into the second cylinder is then ascertained by forming a quotient from the second sampling-value sum and the second count value. In the following, a gas-fill difference between the first and the second cylinder is then ascertained by comparing the first air mass filled into the first cylinder to the second air mass filled into the second cylinder. A first gas-exchange actuator for actuating the intake valve of the first cylinder and a second gas-exchange actuator for actuating the second intake valve of the second cylinder are then controlled on the basis of this gas-fill difference.
This embodiment of the invention advantageously permits control of the two gas-exchange actuators of the first and of the second cylinder on the basis of the actual deviations of the two gas-exchange actuators, namely, on the basis of the gas-fill difference. The gas-fill difference can thus be minimized to improve the smooth running properties.
The device of the present invention includes a sampling device for ascertaining sampling values of a detection signal of a filling sensor by sampling the detection signal at a sampling rate; a first determination device for determining a first detection interval for a first cylinder of the plurality of cylinders; a first summing unit for summing sampling values which are within the first detection interval for the first cylinder, for ascertaining a first sampling-value sum; a first counter for counting a number of sampling values which are within the first sampling interval, for ascertaining a first count value; and a first divider for ascertaining a first air mass filled into the first cylinder by forming a quotient from the first sampling sum and the first count value.
The advantages attained with the invention lie particularly in the fact that only one filling sensor is required to ascertain a first air mass which was filled into the first cylinder. In other words, only one detection signal of one sensor is sampled, and the first air mass is ascertained on the basis of this signal. Consequently, the device of the present invention has a simple design which can be produced inexpensively. In addition, since only one filling sensor is provided, it is not necessary to take into account deviations between a plurality of filling sensors.
In one advantageous refinement of the invention, the first detection interval is variable.
Thus, a xe2x80x9cdetection windowxe2x80x9d can be formed in such a way that, for example, only an opening or closing of the valve is considered, and therefore it is possible to check whether tolerances of the valve or of the gas-exchange actuator are particularly great or particularly small upon opening or upon closing of the valve.
Another advantageous embodiment of the invention further includes a sampling-rate determination device for determining a sampling rate; a second determination device for determining a second detection interval for a second cylinder of the plurality of cylinders; a second summing unit for summing sampling values within the first detection interval, for ascertaining a second sampling-value sum; a second counter for counting a number of sampling values within the second detection interval, for ascertaining a second count value; a second divider for ascertaining a second air mass filled into the second cylinder by forming a quotient from the second sampling sum and the second count value; a first comparator for ascertaining a gas-fill difference between the first and the second cylinder by comparing the first air mass to the second air mass; and a control device for controlling a first gas-exchange actuator of the first cylinder and a second gas-exchange actuator of the second cylinder on the basis of the gas-fill difference.
This embodiment of the present invention advantageously permits triggering of the two gas-exchange actuators on the basis of the gas-fill difference between the two cylinders, so that gas-fill differences can be offset. This allows excellent smooth running of the internal combustion engine. A decrease in noise and pollutant emissions is also thereby attainable, as well as a reduction in fuel consumption. Thus, a fresh gas and residual exhaust gas charge which fluctuates from cylinder to cylinder can be compensated by cylinder-individual triggering of the gas-exchange actuators.